The present invention relates to a surface treated steel sheet for a battery container, a method of producing a surface treated steel sheet for a battery container, a battery container and a battery using the steel sheet.
With the increase of the demand for alkaline manganese batteries used for small-sized domestic electric devices such as shavers, the demand for the improved performance of these batteries has been increased. Above all, the conductivity of electrons between the inner surface of a battery container which is the positive electrode and the positive electrode active material inserted into a battery should be made excellent so as to achieve the improved battery performance. For that purpose, a lot of proposals are disclosed as follows:
A method of applying a coating having excellent conduction on the inner surface of a container (Laid Open Japanese Patent Sho. 58-48361 and Laid Open Japanese Patent Sho. 59-160959),
a pre coated steel sheet in which adhesion of the coating is improved (Laid Open Japanese Patent Hei. 6-342653 and Laid Open Japanese Patent Hei. 8-287885),
as the other methods for improving adhesion of the coating,
a method of forming concave and convex portions on the inner surface of a container (Laid Open Japanese Patent Sho. 59-209056), and
a method of causing roughened surface or cracks on the inner surface of a container (Laid Open Japanese Patent Hei. 9-306439).
However, the conventional method of pre coated steel sheet or the like, in which a coating having excellent conduction (conductive coating) is applied on the one side of a steel sheet to become the inner surface of a container, is not favorable because the conductive coating is easily peeled off when the pre coated steel sheet is formed into a battery container, while a method of applying a coating on the inner surface of a container causes the increase of the process which produces an increase in cost.
Considering these problems, the objective of the present invention is to produce an improved surface treated steel sheet having more excellent conductance with the positive electrode active material.
Further, another objective the present invention is to produce a battery container and a battery using the surface treated steel sheet.
The surface treated steel sheet for a battery container is characterized in that a graphite dispersed nickel plating layer is formed on at least one surface of the surface treated steel sheet, that is to become the inner side of a battery container.
The surface treated steel sheet for a battery container is characterized in that a graphite dispersed nickel alloy plating layer is formed on at least one surface of the surface treated steel sheet, that is to become the inner side of a battery container.
In these surface treated steel sheets it is preferable that the alloy plating layer be any layer of nickel-cobalt alloy, nickel-cobalt-iron alloy, nickel-manganese alloy, nickel-phosphorus alloy or nickel-bismuth alloy.
In these surface treated steel sheets it is preferable that a diffusion layer be formed under the plating layer.
Further, in these surface treated steel sheets it is preferable that any layer of mat nickel plating, semi-gloss nickel plating, gloss nickel plating, nickel-cobalt alloy, nickel-cobalt-iron alloy, nickel-manganese alloy, nickel-phosphorus alloy or nickel-bismuth alloy be formed under the plating layer.
In these surface treated steel sheet it is preferable that the content of graphite in the plating layer is 0.1 to 25 weight %.
The method according to the present invention is characterized in that a plating layer is formed on at least on one side to become the inner side of a battery container using a plating bath including nickel salt, surface active agent and graphite powder.
The method according to another embodiment of the present invention is characterized in that a plating layer is formed on at least one side to become the inner side of a battery container using a plating bath including one kind or more of metal salt consisting of cobalt salts, iron salts, manganese salts, salts of phosphorus anions and bismuth salts, nickel salts, along with a surface active agent and graphite powder.
The battery container according to another embodiment of the present invention is characterized in that it is produced from the above-mentioned surface treated steel sheet.
The battery according to the present invention is characterized in that it uses the battery container produced from any surface treated steel sheet produced as described in the present specification.
The present invention will be described in detail below.
[Steel Sheet Used]
A cold-rolled steel sheet based on a plain steel, particularly based on a continuously casted low carbon aluminum killed steel is used as a steel sheet for the present invention. In addition, a hyper low carbon steel having 0.003 wt % or less of carbon content, an anti aging steel that is the hyper low carbon steel including metals such as niobium, titanium and so on, or a stainless steel sheet including 3 to 18 wt % of chromium content is also available.
[Substratum Nickel Plating]
In the surface treated steel sheet for a battery container, it is preferable that a layer of nickel plating be formed on a steel sheet. Hereinafter, the nickel plating is described as substratum nickel plating. The objective of the formation of the substratum nickel plating layer is to produce enough corrosion resistance even after the plated steel sheet is formed into a battery container to prevent corrosion of the steel sheet.
As a plating bath for the substratum nickel plating, the bath used in the usual nickel plating such as watts bath, sulfamate bath, borofluoride bath or chloride bath can be applied to the present invention. There are 2 types of nickel plating, electrolytical plating and electroless plating. While electroless plating is available, in general electrolytical plating is easier since the bath composition and the plating thickness can be controlled. The current density of 3 to 80 A/dm2 is applied in electrolytical plating and it is preferable to stir the bath by air bubbling for forming a plating layer having a uniform thickness. Furthermore, pH of the bath is preferably in the range of 3.5 to 5.5, and the bath temperature is preferably in the range of 40 to 60xc2x0 C.
In the present invention, as a substratum nickel plating, any type of mat plating without the use of organic additives, semi-gloss plating or gloss plating using organic additives can be applied. The quantity of the substratum nickel plating layer is preferably 0.5 to 5 xcexcm. The quantity of less than 0.5 xcexcm produces insufficient covering of the steel sheet, which can not cause enough corrosion resistance that is the objective of the substratum nickel plating, while a layer of more than 5 xcexcm saturates the covering effect as well as causing unfavorable economical effect. It is preferable to form the substratum nickel plating on both surfaces of a steel sheet in view of securing corrosion resistance. It is preferable to form the plating having a thickness of about 1 to 3 xcexcm on the one surface to become the inner surface of a container, while it is preferable to form the plating having a thickness of about 1 to 4 xcexcm on the other surface to become the outer surface of a container. It is preferable that the plating thickness on the surface to become the outer surface of a container is a little greater than that on the surface to become the inner surface of a container in view of preventing rust occurrence of a battery container.
Further, as the other substratum plating, for example nickel-cobalt plating, nickel-cobalt-iron plating, nickel-manganese plating, nickel-phosphorus plating or nickel-bismuth plating based on alloy plating bath comprising nickel and any of cobalt, manganese, iron, phosphorus or bismuth can also be available. As a plating bath, known sulfate bath, sulfamate bath or the like can be used. The thickness of the plating might be in the same range as that of the substratum nickel plating.
[Formation of Diffusion Layer]
While the substratum nickel plating layer may be the one as plated, it is preferable to transform the nickel plating layer entirely or partially into a diffusion layer by a heat treatment after plating. The formation of the diffusion layer prevents peeling-off of the nickel plating layer from a steel substrate.
The heat treatment is preferably carried out in an anti-oxidation atmosphere or in a reducing protective atmosphere to prevent the formation of a oxidized film on the surface of the diffusion layer. As an anti-oxidation atmosphere, a so-called inert gas such as nitrogen, argon or helium is preferably used, while as a reducing gas hydrogen or ammonia cracking gas (75% of hydrogen and 25% of nitrogen) is preferably used. As a method for the heat treatment, any of a box annealing or a continuous annealing is available. In case of the box annealing, the temperature for the heat treatment is preferably 450xc2x0 C. or more. The duration for the heat treatment is shorter in the continuous annealing, while it is rather longer in the box annealing. Generally, it is preferably 30 seconds to 2 minutes for the continuous annealing and 6 to 15 hours for the box annealing.
[Formation of Graphite Dispersed Nickel (Alloy) Plating Layer]
This graphite dispersed nickel plating layer is formed on a side corresponding to the inner surface of a battery container. As a plating bath, one based on a nickel plating bath in which graphite is dispersed (thereby a graphite dispersed nickel plating layer is formed) or another one based on alloy plating bath comprising a metal excluding nickel, such as cobalt, manganese, iron, a salt of a phosphorus compound or bismuth, and nickel in which graphite is dispersed (thereby a graphite dispersed nickel alloy plating layer is formed) is used. However, as metals such as molybdenum, antimony, arsenic, chromium or the like and semi metals have possibility to generate gas in the battery or to drop the voltage, it is preferable to avoid the use of a bath containing these metals or semi metals. The use of a plating bath in which graphite having excellent conductance is dispersed causes the improvement in the collecting ability of electricity of the plating layer with the positive electrode active material by dispersedly codepositing graphite in the plating layer accompanied with formation of the plating layer and by exposedly scattering graphite on the plating layer. In contrast to the surface of the steel sheet/nickel plating layer which is conventionally used, the surface of the nickel plating layer/graphite dispersed nickel layer of the present invention has a greater number of concave and convex portions and a greater surface area, which causes a small contact resistance. Further, in contrast to the conventional combination of steel sheet/nickel plating layer/graphite layer, the combination of steel sheet/nickel plating layer/graphite dispersed nickel plating layer/graphite layer of the present invention has a smaller resistance. It is also affected by the smaller resistance at the interface of graphite dispersed nickel plating layer/graphite layer of the present invention in contrast to the interface of nickel plating layer and graphite layer.
While either natural graphite or synthetic graphite is available for the present invention, finely crushed graphite having 50% cumulative diameter of 10 xcexcm or less is favorably used. Further, graphite having 50% cumulative diameter of 5 xcexcm or less is more favorably used, because the use of graphite having too large a particle diameter compared with the plating thickness causes easy peeling-off of the adhered graphite.
Further, it is also favorable to use graphitized carbon black. Graphitized carbon black is carbon black which is graphitized and has an extremely fine particle diameter of about 0. 1 xcexcm or less.
Since graphite has a hydrophobic surface, it is hard to disperse graphite in the plating bath by mere stirring. Therefore, the graphite is forcedly dispersed using a surface active agent dispersing agent of graphite). While any type of surfactant, including cationic, anionic, nonionic or amphoteric can be used as the dispersing agent of graphite, in order to obtain excellent adhesion of the plating layer to the steel sheet and to prevent embrittlement of the plating layer, it is preferable to use an anionic surface active agent as the dispersing agent of graphite for the present invention. Of anionic surface active agents, surfactants based on benzene sulfonic acids or sulfate esters, for example, sodium alkyl sulfate, sodium dodecyl benzene sulfonate, sodium xcex1 olefin sulfonate, sodium alkyl naphthalene sulfonate, sodium 2 dialkyl sulfosuccinate or the like are more favorable as the dispersing agent of graphite for the present invention.
Dispersing of the fine graphite into the plating bath is carried out as follows: Namely, the graphite powder is mixed with the dispersing agent of graphite diluted with a certain amount of water, then finally dispersed using an emulsifying mixer such as homogenizer or ultrasonic cleaner. In this case, it is effective for dispersing to moisten the graphite powder with a small amount of alcohol or the like. Thus, after being fully dispersed, the graphite is added into the plating bath under stirring. The blending amount of dispersing agent to graphite is preferably 0.5 to 10 wt %. The content of graphite in the plating bath is preferably controlled as 1 to 100 g/L at the end. While the content of less than 1 g/L causes to small graphite content in the plating, which produces insufficient improvement in conductance between the inner surface of a battery container and positive electrode active agent, the content of more than 100 g/L causes poor fluidity of the plating solution or adhesion of graphite powder around the plating apparatus, which is apt to cause some troubles. Further, 2 to 10 ml/L of dispersing agent is previously added into the plating solution for preventing aggregation of graphite particles.
After dispersing graphite powder in the plating solution, it is preferable that the graphite is always being dispersed in the plating bath by both methods, of which one is to circulate the plating solution to the bottom portion of the electrolysis tank using a circulation pump, and another is to stir the plating solution by blowing air through the micro holes perforated at the bottom portion of the electrolysis tank. Under the favorable dispersion, graphite can be scattered in the plating layer at the content of 0.1 to 25%. Above all, it is favorable to be scattered at the content of 1 to 10%. Incidentally, it is favorable to use the lower current density for graphite dispersed plating layer having greater amount of graphite content.
[Formation of Battery Container]
A battery container is preferably formed by drawing and ironing forming, that is so-called DI forming (drawing and ironing) or DTR forming (drawing thin and redraw). In case of DI forming, a shallow drawn cup made of a surface treated thin steel sheet having slightly larger diameter than that of a battery container is firstly prepared. After that, the cup is provided to multi step ironing dies which are coaxially installed in the manner that the ironing diameter of them becomes smaller one by one, and of which final step ironing die has the ironing diameter corresponding to the outer diameter of the battery container, then it is successively passed through the ironing dies taking care not to cause a waist by pressing using a punch of which the head has rounded shoulder.
In case of DTR forming, a shallow drawn cup is firstly prepared as in case of DI forming, then successively redrawn into a redrawn cup having smaller diameter and taller height than those of the first shallow cup. Namely, in the redrawing, the redrawn cup is held by a nest ring inserted into the redrawn cup and a redrawing die provided under the redrawn cup, a redrawing punch is coaxially installed so as to shuttle in the nest ring, and redrawing dies having varied drawing diameter are successively used. In case of further necessity, the battery container may be formed by the other forming method.
[Production of Alkaline Manganese Battery]
The positive electrode mix is produced by mixing manganese dioxide, carbon and alkaline water solution. It is preferable to use electrolytical manganese dioxide of high purity as manganese dioxide.
The characteristics required for the graphite powder are high purity, chemical steadiness, and excellency in conductance, formability into mix and storage ability of elecrtrolyte. As an example of graphite powder having the above-mentioned required characteristics, acetylene black, several kinds of denatured carbon black such as graphitized carbon black or synthetic graphite powder are listed.
The positive electrode mix is produced by mixing electrolytical manganese dioxide and graphite powder at the preferable weight ratio of 20/1 to 10/1, further adding potassium hydroxide water solution and mixing by some suitable method.
Further, in case of necessity, it is also preferable for excellent conductance between the battery container and the positive electrode mix, for example, to coat a mixture comprising graphite powder, thermosetting resin and organic solvent such as methyl ethyl ketone on the inner surface of a battery container using spraying and so on, and dry it.
Next, the afore-mentioned mix is pressed in a mold to form a prescribed doughnut shaped mix pellets, and then the mix pellets are inserted and pressed to the inside of the battery container. Further, the prescribed portion under the open edge portion of the battery container is previously processed by neck-in forming for mounting the negative electrode board on which the negative electrode collector rod is spot welded to the battery container.
The separator used in a battery, of which the objective is to prevent the mutual migration of the particles of the negative electrode active material and the positive electrode active material and separate the reaction product formed at the negative electrode from the positive electrode by itself so as to prevent short circuit and self discharge in the battery, is made of an alkali resistant fibrous material or unwoven cloth. For instance, synthetic resin material such as vinylon, polyolefin, polyamide or the like, linter pulp having 98% or more of xcex1 cellulose content, mercerized wood pulp, regenerated cellulose or the like can be used.
These fibrous separators are inserted in the battery container along the inner circumference of the positive electrode mix pellets pressed to the battery container, and then the negative electrode gel comprising potassium hydroxide water solution in which zinc particle and zinc dioxide are dissolved is inserted in the battery container. In this case, atomized zinc particle having a center diameter of around 200 xcexcm is preferably used. Moreover, starch, cellulose derivative, polyacrylate or the like can be used as a gelling material.
After this is inserted into the battery container, a gasket of insulating material is mounted on the negative electrode board, and then a lid is caulked. Thus, an alkaline manganese battery is completed.
Embodiments according to the present invention will be described in detail below.